Alumina-pillared montmorillonite

Cationic clays have also been used as supports for Cu. Cu-doped alumina-pillared montmorillonites have been employed in the oxidation of toluene and of xylenes with H2C>2. The pillaring and the Cu exchange are performed under acidic conditions at pH 2 and 3.5, respectively. It is unclear whether the Cu2+ remains fully associated with the clay in the presence of H2O2, which is itself acidic. Moreover, the reactions are unselective mixtures of ring-hydroxylated and side chain-oxidized products are obtained (180). 

The latter approach was adopted by Trillo et al. (1993) in their study of the effect of thermal and hydrothermal treatment on the accessible microporosity of alumina-pillared montmorillonite. This work revealed that X-ray measurements of the dmi spacing taken alone may give a misleading impression of the thermal stability of the PILC micropore structure. For example, after heat treatment of the Al-PILC at 300°C, it was found that the apparent micropore volume available for nitrogen adsorption amounted to only c. 30% of that indicated by the dm spacing. 

In a study of the porosity of alumina-pillared montmorillonites (Al-PILCs), Zhu et al. (1995) have obtained values of the mean slit-width of 0.8-0.9 nm from the volume/surface ratio. In this case, the nitrogen adsorption values were in agreement with the corresponding dm values of c. 0.8 nm. However, effective micropore volumes obtained from the nitrogen isotherms and from water sorption data were significantly different and it was suggested that the density of the sorbed water was lower than that of liquid water. 

Z. H. Ge, D. Y. Li, T. J. Pinnavaia, Preparation of alumina-pillared montmorillonites with high thermal stability, regular microporosity and Lewis/Bronsted acidity. Microporous Mater., 3(1994), pp. 165-175. 

The microporosity of a new tubular silicatelayered silicate nanocomposite formed by the intercalation of imogolite in Na -montmorillonite has been characterized by nitrogen and m-xylene adsorption. The nitrogen adsorption data yielded liquid micropore volume of -0.20 cm g as determined by both the t-plot and the Dubinin-Radusikevich methods. The t-plot provided evidence for a bimodal pore structure which we attributed to intratube and intertube adsorption environments. The m-xylene adsorption data indicated a much smaller liquid pore volume (-0.11 cm g ), most likely due to incomplete filling of intratubular pores by the planar adsorbate. The FTIR spectrum of pyridine adsorbed on the TSLS complex established the presence of both Bronsted and Lewis acid sites. The TSLS complex was shown to be active for the acid-catalyzed dealkylation of cumene at 350 C, but the complex was less reactive than a conventional alumina pillared montmorillonite. 

Figure 8. Catalytic dealkylation of cumene at 350 C over the imogolite-montmorillonite TSLS complex and alumina pillared montmorillonite (APM).

Microporous solid selected in this study is alumina pillared montmorillonite (Al-PILM) which exhibits well-defined slit-shaped micropores. Four mesoporous solids are examined Two silica MCM-41 samples prepared with quaternary ammonium surfactants dodecyltrimethylammonium bromide (the main carbon chain of the ammonium has 12 carbon atoms. Cl2) and cetyltrimethylammonium bromide. They are labeled as MCM-41 (Cl2) and MCM-41 (Cl6), respectively. The other two are commercial porous silicas (Kieselgel 60 and silica gel 40 A from Aldrich). 

One of the important catalytic processes based on shape selectivity is the alkylation of biphenyl with propene (Reaction 4). Pinnavaia et al. have shown that mesoporous clays such as K10 and alumina-pillared montmorillonite are more selective than homogeneous acid catalysts, although not as... 

Michott, L. and Pinnavaia, T. (1992) Improved synthesis of alumina-pillared montmorillonite by surfactant modification. Chem. Mater., 4, 1431-1433. 

In 1990, Choudary [139] reported that titanium-pillared montmorillonites modified with tartrates are very selective solid catalysts for the Sharpless epoxidation, as well as for the oxidation of aromatic sulfides [140], Unfortunately, this research has not been reproduced by other authors. Therefore, a more classical strategy to modify different metal oxides with histidine was used by Moriguchi et al. [141], The catalyst showed a modest e.s. for the solvolysis of activated amino acid esters. Starting from these discoveries, Morihara et al. [142] created in 1993 the so-called molecular footprints on the surface of an Al-doped silica gel using an amino acid derivative as chiral template molecule. After removal of the template, the catalyst showed low but significant e.s. for the hydrolysis of a structurally related anhydride. On the same fines, Cativiela and coworkers [143] treated silica or alumina with diethylaluminum chloride and menthol. The resulting modified material catalyzed Diels-Alder reaction between cyclopentadiene and methacrolein with modest e.s. (30% e.e.). As mentioned in the Introduction, all these catalysts are not yet practically important but rather they demonstrate that amorphous metal oxides can be modified successfully. 

Many other materials, including synthetic aluminas, aluminum carbonates, aluminum silicates, magnesium silicates, various forms of attapulgite and sepiolite (81-83), alumina-pillared acid-activated montmorillonite (84), synthetic mica mont-morillonite, HY-zeolite, zirconium phosphate (85), mica, kaolin, and synthetic hectorite (86), have been evaluated for their ability to purify virgin fats and oils, but none were as good as acid-activated bentonite. 

Titanium-pillared montmorillonites (Ti-PILC) modified with tartrates were described as heterogeneous Sharpless epoxidation catalysts [33] as well as for the oxidation of aromatic sulfides [34]. Metal oxides modified with histamine showed modest efficiencies for the kinetic resolution of activated amino acid esters (kj /k5 2) [35]. Silica or alumina treated with diethylaluminium chloride and menthol catalyzed the Diels-Alder reaction between cylopentadiene and methacrolein with modest enantioselectivities of up to 31% ee [36]. ZeoHte HY, modified with chiral sulfoxides had remarkable selectivities for the kinetic resolution of 2-butanol (k /kj =39) but unfortunately the catalyst is not very stable... 

Figure 5. A comparison of catalytic activities using a light cycle oil over deactivated catalysts based on pillared montmorillonite, pillared rectorite, NaY and an amorphous silica-alumina FCC.

Because acidified titanium oxide is the catalyst usually employed commercially for the transformation of 1 into 2 [8] there has been much investigation of this catalytic system [9]. A 1995 paper by Stefanis et al. [10] reported an investigation of the reaction of 1 in several alumina-pillared clays (PILCs montmorillonite- and beidellite-based, and their and Ca" -exchanged congeners) under Lewis acid conditions (solid is activated by heat to remove all water). The results were compared with those obtained by use of medium-pore zeolites USY, NH4+-ZSM-5, and H-mordenite. Conversion to 2 > 50% was always observed. The aim of the work was to clarify differences between site availability and acidity for the two types of solid. 

Preparation, physicochemical characterization and catalytic properties of vanadium-doped alumina- and titania-pillared montmorillonites... 

Procedures leading to preparation of vanadium-doped alumina- and/or titania-pillared montmorillonites are described and physicochemical characterization (chemical analysis, XRD, BET, ESR) of the products is provided. Results show that introduction of vanadium into the pillared montmorillonites leads to a rigid association of the dopant with pillars, irrespective of the method of preparation. The mode of vanadyl attachment in alumina-pillared samples does not depend on the mode of preparation, while in titania-pillared montmorillonite it does. Certain degree of delocalization of the unpaired electron into ligands and increased in-plane 7t-covalent bonding is observed for vanadyl ions present in the co-pillared (V-Ti)-PILC samples which also show particularly high activity in catalytic ammoxidation of m-xylene to nitrile product, as monitored by IR. A hypothesis is advanced that this effect is due to the unique character of vanadyl species present in these catalysts. 

This paper describes preparation, physicochemical characterization and catalytic properties of a series of vanadium-doped alumina- and titania-pillared montmorillonites obtained by various methods The aim of this work was to investigate the influence of the preparation procedure and pretreatment on location of vanadium dopant within the PILC structure and to correlate the physicochemical characteristics of tiie samples with their catalytic activity in ammoxidation of m-xylene. 

ESR data presented in detail elsewhere [7] show that all vanadium-doped alumina-and/or titania-pillared montmorillonite samples contain immobilized vanadyl ions bound to the pillars. 

Fig. la shows typical ESR spectra of alumina-pillared samples obtained by different methods. Differences in the ESR parameters of vanadyl species introduced by exchange with uncalcined or calcined pillars are very small indicating that similar vanadium species are formed irrespective of the preparative procedure. Also in the case of titania-pillared montmorillonite the ESR parameters of vanadyl ions deposited onto uncalcined (V(Ti)-PILC) and calcined (V-(Ti-PILC)) pillars are similar In the co-pillared (V-Ti)-PILC sample, however, a vanadyl spectrum with different parameters is observed (e g. smaller value of Aj, Fig. lb) pointing to a different character of vanadyl-pillar bonding in this case Analysis of the ESR parameters [7] shows that in the co-pillared samples the unpaired electron is partially delocalized into ligands and the vanadyl species posses increased inplane 7t-covalent bonding. 

Another technique for modifying PILCs, found to be useful to improve the adsorption properties, is the incorporation of specific cations in the porous strac-ture of pillared clays, serving as specific adsorption sites in certain applications. It is known that the cation exchange capacity (CEC) of montmorillonite greatly decreases after pillaring with alumina pillars, due to the nonexchangeable H cations in the clay structure that are formed during the calcination. It is, however, possible to restore the CEC of a PILC in two ways (82,83). 

A comparative study of the structure developed by two alumina-pillared clays using the results of adsorption from nitrogen, hydrogen and carbon dioxide is reported. The samples considered come from a montmorillonite and a saponite that have been treated with solutions of hydrolysed aluminium. The solids resulting of the intercalation have been calcined at 473, 623 and 773 K for 4 h. The textural properties of the samples derived from the results of nitrogen adsorption at 77 K, show a decrease of the specific surface area and the micropore volume as the temperature of calcination increases. Results from hydrogen adsorption indicate the presence of micropores of widths smaller than the diameter of the molecule of nitrogen, while carbon dioxide adsorption reveals the presence of specific sites of chemisorption. 

In Fig. 3.58 is shown the integrated intensities of the band at 1540 cm" for pillared beidellite and montmorillonite against the outgassing temperature. Increasing the calcination temperature prior to pyridine adsorption results in a steep drop in the proton content in the case of pillared montmorillonite, while pillared beidellite keeps its acidity. The steep drop of the Bronsted acid sites observed for pillared montmorillonite was attributed to the fact that, upon thermal activation, the protons migrate into the octahedral layer of the clay, where they induce a premature dehydroxylation. Thus, the acidity is mainly of the Lewis type for samples treated at higher temperatures. A similar result was reported also for bentonites pillared with alumina clusters. ... 

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